The present invention relates to a nonaqueous electrolyte secondary battery.
Recently, a lithium ion secondary battery is put on the market as a nonaqueous secondary battery used in a portable electrical appliance such as a portable telephone. The battery comprises a positive electrode containing lithium cobalt oxide (LiCoO2), a negative electrode containing a graphite material or a carbonaceous material, a nonaqueous electrolyte prepared by dissolving a lithium salt in an organic solvent, a separator formed of a porous membrane, and a metal jacket formed of a cylindrical or rectangular metal case.
With decrease in the thickness of a portable appliance, it is requested that the nonaqueous secondary battery be made smaller in thickness and lighter in weight. In order to decrease the thickness of the lithium ion secondary battery to 4 mm or less, it is necessary to decrease the thickness of the metal case used as a jacket.
However, if the thickness of the metal case is decreased, it is difficult to form the metal case itself. Thus, it was substantially difficult to put a lithium ion secondary battery having a thickness not larger than 4 mm to a practical use.
On the other hand, U.S. Pat. No. 5,437,692 discloses a lithium ion secondary battery, in which an electrode group consisting of a positive electrode, a negative electrode and a gel-like polymer electrolyte arranged between the positive and negative electrodes is sealed in a battery housing. The inventions disclosed in these U.S. Patents are intended to improve the cycle life by increasing the mobility of the lithium ions. Also, the details of the battery housing are not described at all in these U.S. Patents.
However, the secondary battery disclosed in this prior art tends to give rise to an abnormal heat generation or temperature elevation because of an external short-circuiting, an over-charging or leaving to stand under high temperatures not lower than 130xc2x0 C., leading to a dangerous situation that the viscosity of the polymer electrolyte layer is lowered so as to bring the positive electrode into contact with the negative electrode. This internal short-circuiting tends to take place so as to bring about a danger of explosion.
An object of the present invention is to provide a nonaqueous electrolyte secondary battery with an improved safety.
According to a first aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising an electrode group including a positive electrode, a negative electrode, and a porous separator interposed between the positive electrode and the negative electrode, the pores of the porous separator being closed when heated; a nonaqueous electrolyte held by the porous separator; and a jacket formed of a sheet including a thermoplastic resin layer forming at least a portion of the inner surface, having the electrode group housed therein, and having mutually facing regions of the thermoplastic resin layer heat-sealed to each other to seal the electrode group therein; wherein the positive electrode, the negative electrode and the separator are made integral, and the thermoplastic resin layer has a melting point higher than a pore-closing initiating temperature of the porous separator at which the pores of the separator begin to be closed.
According to a second aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising an electrode group including a positive electrode, a negative electrode, and a separator made of a porous sheet interposed between the positive electrode and the negative electrode, the porous sheet being made of at least one kind of a material selected from the group consisting of polyolefin, cellulose and polyvinylidene fluoride, having an air permeability not higher than 600 seconds/100 cm3, and a having a thickness of 5 to 30 xcexcm; a nonaqueous electrolyte held by the separator; and a jacket formed of a sheet including a thermoplastic resin layer forming at least a portion of the inner surface, having the electrode group housed therein, and having mutually facing regions of the thermoplastic resin layer heat-sealed to each other to seal the electrode group therein, the negative electrode and the separator are made integral, and the thermoplastic resin layer has a melting point higher than a pore-closing initiating temperature at which the pores of the separator begin to be closed.
According to a third aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising an electrode group including a positive electrode, a negative electrode, and a separator made of a porous sheet interposed between the positive electrode and the negative electrode, the porous sheet being made of at least one kind of a material selected from the group consisting of polyolefin, cellulose and polyvinylidene fluoride; a nonaqueous electrolyte held by the separator; and a jacket formed of a sheet including a polyolefin layer forming at least a portion of the inner surface, having the electrode group housed therein, and having mutually facing regions of the polyolefin layer heat-sealed to each other to seal the electrode group therein; wherein the positive electrode, the negative electrode and the separator are made integral, and the polyolefin layer has a melting point higher than a pore-closing initiating temperature at which the pores of the separator begin to be closed.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.